Fast transition, flat pulse generator

ABSTRACT

A pulse generator utilizes a pair of current sources acting through a pair of Schottky diode switches to provide load currents in opposite directions through a load resistor to produce an output voltage across the load resistor of magnitude proportional to the difference between the load current magnitudes. The pulse generator output voltage is abruptly driven to zero potential by applying balanced, rapid slewing voltage pulses to the Schottky diode switches, causing the switches to simultaneously divert the load currents from the load resistor. The balanced voltage pulses are developed at the terminals of a step-recovery diode by switching the step-recovery diode from forward to reverse bias state.

BACKGROUND OF THE INVENTION

The present invention relates in general to pulse generators utilizingdiode switches to switch load currents through a load resistor toproduce a stepped output voltage.

The bandwidth of an amplifier (or any other electronic device) can bedetermined by measuring the output of the amplifier in response to arange of input signal frequencies but such a method can be timeconsuming. Fortunately the bandwidth of an amplifier can be more quicklydetermined by observing its response to a square wave signal since therise time of the amplifier output voltage in response to an abrupttransition in an input voltage is inversely proportional to theamplifier bandwidth. The accuracy of the measurement depends on theabruptness of the input voltage transition provided by the square wavesignal and also on the flatness of the square wave signal before andafter the transition.

One method of producing a relatively fast voltage transition utilizes aSchottky diode switch to divert a constant current away from a loadresistor. As illustrated in FIG. 1, the diode switch comprises a pair ofSchottky diodes Da and Db having cathodes connected to a current sourceIs. The anode of diode Db is connected to ground through a load resistorR_(o) while the anode of diode Da is connected to a negative voltage -Vthrough another resistor R and to a source of positive voltage +Vthrough a high speed switch S. When switch S is open, the -V sourceforward biases diode Db and reverse biases diode Da such that current Isis drawn through resistor R_(o) to produce a negative output voltage Vo.When switch S is closed, voltage source +V forward biases diode Da andreverse biases diode Db so that current Is is supplied from +V throughdiode switch S and diode Da rather than from ground through resistorR_(o) and diode Db. As the rise time of the leading edge of the pulseapplied to diode Da produced by closing switch S is decreased, thetransition of Vo from a positive voltage to a zero voltage becomes moreabrupt. However diodes Da and Db include inherent capacitances whichdifferentiate the square wave pulse from switch S1 to produce a smallcurrent which is fed through diodes Da and Db to the load resistor R_(o)and this current causes the output voltage Vo to ring following atransition. As the rise time of the leading edge of the pulse applied toDa is increased, the amount of ringing in Vo after Vo switches state isalso increased. Thus the switching speed of this circuit is limited bythe amount of ringing that can be tolerated in Vo following a statetransition.

What is needed is a pulse generator for producing an abrupt statetransition in an output voltage wherein the output voltage is flatfollowing the state transition.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a pulse generatorutilizes a pair of current sources acting through a pair of Schottkydiode switches to provide load currents of opposite polarity through aload resistor. The load currents produce an output voltage across theload resistor of magnitude proportional to the difference between theload current magnitudes. The pulse generator output voltage is abruptlydriven to zero potential by applying balanced, rapid slewing voltagepulses of opposite polarity to the Schottky diode switches, causing theswitches to simultaneously divert the load currents from the loadresistor. Since the voltage pulses are balanced and of oppositepolarity, any ringing disturbance to the zero potential of the outputvoltage following an output voltage transition resulting from currentsfed through one diode switch to the load resistor is matched by an equaland opposite disturbance resulting from currents of opposite polarityfed through the other diode switch. Therefore the output voltage remainsflat following transition to zero potential.

In accordance with another aspect of the invention, the magnitude of thecurrent provided by each current source is adjustable. Since themagnitude of the output voltage produced by the pulse generator isproportional to the difference between the two opposite polaritycurrents, the output voltage of the pulse generator prior to transitionto zero potential can be adjusted over a range of positive and negativevalues so that the transition to zero potential may be selectivelyeither a rising or falling edge.

In accordance with a further aspect of the invention, the balanced,rapid slewing voltage pulses are developed at the terminals of astep-recovery diode by switching the step-recovery diode from forward toreverse bias state. The step-recovery diode switches rapidly to areverse bias state so that the balanced voltage pulses have abrupt edgescausing the diode switches to abruptly terminate the output voltage witha rise time on the order of a few picoseconds.

It is accordingly an object of the invention to provide a new andimproved pulse generator which provides a voltage pulse having an abrupttransition to zero potential from selectively either positive ornegative potentials.

It is another object of the invention to provide a new and improvedpulse generator which eliminates ringing in its output voltage followingtransition zero potential.

The subject matter of the present invention is particularly pointed outand distinctly claimed in the concluding portion of this specification.However, both the organization and method of operation, together withfurther advantages and objects thereof, may best be understood byreference to the following description taken in connection withaccompanying drawings wherein like reference characters refer to likeelements.

DRAWINGS

FIG. 1 is a schematic diagram of a pulse generator according to theprior art; and

FIG. 2 is a schematic diagram of a pulse generator according to thepresent invention.

DETAILED DESCRIPTION

Referring to FIG. 2, there is depicted in schematic diagram form a pulsegenerator 10 according to the present invention adapted to produce anoutput voltage Vo of adjustable magnitude and polarity across a loadresistor R_(o) and to abruptly drive the output voltage to zeropotential after a control current Ic produced by a current source 11 isapplied to the pulse generator. When current source 11 is turned off,the output voltage Vo is produced in response to two currents I1 and I2transmitted in opposite directions through the load resistor R_(o) fromcurrent sources 12 and 14 respectively. Thus the magnitude of Vo isproportional to the difference in magnitudes between I1 and I2 and Vomay be either positive or negative with respect to ground potential,depending on whether I2 is larger than or less than I1.

The load resistor R_(o) is grounded at one end while its other end isconnected to the anode of a Schottky diode D2 and a cathode of anotherSchottky diode D4. A resistor R1 connects the cathode of diode D2 tocurrent source 12 while a resistor R2 connects the anode of diode D4 tocurrent source 14. When diodes D2 and D4 are forward biased, currentsource 12 draws current I1 from ground through the load resistor R_(o),diode D2, and resistor R1 while current source 14 transmits current I2to ground through the series connected resistor R2, diode D4 and loadresistor R_(o). The magnitude of Vo is proportional to the differencebetween I1 and I2 and the magnitudes of I1 and I2 are adjustable. Vowill be positive with respect to ground if the magnitude of I2 exceedsthe magnitude of I1 and negative if the magnitude of I1 exceeds themagnitude of I2.

The cathode of diode D2 is also attached to the cathode of anotherSchottky diode D1. Diodes D1 and D2 form a diode switch 16 which cansupply current I1 to current source 12 either from ground through theload resistor R_(o) and diode D2 or from another source via diode D1,depending on which diode is forward biased. The anode of diode D4 isalso connected to the anode of another Schottky diode D3 to form anotherdiode switch 18 which can either transmit current I2 from current source14 to the load resistor R_(o) via diode D4 or divert current I2 awayfrom the load resistor via diode D3, depending on which diode is forwardbiased.

When currents I1 and I2 are to be transmitted through the load resistorR_(o), a voltage V1 at the anode of diode D1 is driven low in order toforward bias diode D2 and to reverse bias diode D1, while a voltage V2at the cathode of diode D3 is driven high in order to forward bias diodeD4 and reverse bias diode D3. Conversely, when currents I1 and I2 are tobe diverted away from load resistor R_(o) in order to drive Vo to zeropotential, voltage V1 is driven high and voltage V2 is driven low sothat diodes D1 and D3 are forward biased and diodes D2 and D4 arereverse biased. Voltages V1 and V2 are balanced in that they are at alltimes of the same magnitude but of opposite polarity.

When voltage V1 changes from a low to a high potential, small inherentcapacitances in diodes D1 and D2 differentiate the V1 voltage, therebyproducing a small ringing current in response to the change in V1 andthis current is supplied to load resistor R_(o) through D1 and D2. Atthe same time, small inherent capacitances in diodes D3 and D4 produceanother small ringing current in response to the change in voltage V2and this other small current is also supplied to load resistor R_(o)through D3 and D4. Since V1 and V2 are balanced and change in oppositephase relation to one another, the small ringing currents supplied tothe load resistor as a result of changes in V1 and V2 will also be ofequal magnitude but of opposite phase and will therefore cancel oneanother. Thus, the use of a pair of diode switches 16 and 18 controlledby balanced control voltages 16 and 18 to switch opposing load currentsI1 and I2 away from load resistor R_(o) permits the pulse generator 10of the present invention to drive the output voltage Vo to a flat groundpotential while eliminating any disturbance to the ground potential ofVo resulting from ringing currents fed through the diode switches 16 and18.

Control voltages V1 and V2 are derived from the voltages appearing atthe cathode and anode of a step-recovery diode D5. The cathode of diodeD5 is connected to the anode of diode D1 through a capacitor C3 and ashort transmission line T1, while the anode of diode D5 is connected tothe cathode of diode D3 through a capacitor C4 and another transmissionline T2. When diode D5 is forward biased, V1 is low and V2 is high.Diodes D1 and D3 are reverse biased, diodes D2 and D4 are forwardbiased, and currents I1 and I2 are transmitted through the load resistorR_(o). When diode D5 is reverse biased, V1 is high and V2 is low. DiodesD1 and D3 are forward biased, diodes D2 and D4 are reverse biased, andcurrents I1 and I2 are diverted away from the load resistor. Astep-recovery diode stores charge while conducting in the forwarddirection, and when the direction of the current through the diode issuddenly reversed, the diode conducts current in the reverse directionfor a short time while the stored charge is removed. When the storedcharge is removed, the step-recovery diode abruptly cuts off the reversecurrent and the reverse bias voltage across the diode rises rapidly,with a rise time on the order of 70-100 picoseconds.

The cathode of diode D5 is also connected to a negative voltage source-E through a resistor R3 while the anode of diode D5 is connected to apositive voltage source +E through another resistor R4. This arrangementproduces a continuous current I3 passing from source +E to source -Ethrough diode D5 in the forward direction so that diode D5 is normallyforward biased. A balanced pair of switchable current sources 20 and 22are adapted to provide a current I4 through diode D5 in the reversedirection. When I4 is off, diode D5 is forward biased by I3 so that V1remains low and V2 remains high. When I4, which is larger than I3, isturned on, diode D5 conducts briefly in the reverse direction and thenabruptly cuts off reverse current flow, rapidly driving V1 high and V2low. The changes in V1 and V2 rapidly change the bias states of thediodes of diode switches 16 and 18 and as a result, the transition ofoutput voltage Vo to ground potential is abrupt, with a transition timeon the order of a few picoseconds.

Current source 20 includes an NPN transistor Q1 and a PNP transistor Q2which form a "quasithyristor" switch 24 controlled by control current Icapplied to the base of Q2. A positive voltage source +E is applied tothe emitter of transistor Q2. The base of Q2 is coupled to a +E sourcethrough a resistor R7 and to the collector of transistor Q1. The base oftransistor Q1 is connected to ground through a resistor R9, to anegative voltage source -E through a resistor R10, and to the collectorof transistor Q2. A resistor R5 couples the emitter of transistor Q1 toa negative voltage source while an inductor L1 couples the transistor Q1emitter to capacitor C3. The current I4 output of current source 20 istransmitted to capacitor C3 through inductor L1.

When Ic is off, the base of transistor Q2 is high and transistor Q2 isoff. The base of transistor Q1 is held at a voltage determined by thedivider R9, R10 so that transistor Q1 is conducting a constant currentdetermined by R5. This current produces a voltage drop across resistorR7 which does not turn on Q2. In steady state, I4 is zero. The -E voltsource acting through resistor R5 pulls V1 down to reverse bias diode D1and to forward bias diode D2 so that current I1 is supplied throughresistor R_(o).

When current Ic is turned on, the base of transistor Q2 is pulled lowand transistor Q2 turns on. A positive-going voltage pulse appears atthe emitter of transistor Q1 and this pulse is integrated by inductor L1to produce current ramp I4 which is transmitted through capacitor C3 andthrough diode D5 in the reverse direction. The magnitude of current I4by far exceeds the magnitude of current I3 and after its stored chargeis removed, diode D5 abruptly switches off, driving voltage V1 high.With V1 high, diode D1 is forward biased and diode D2 is reverse biased.Current I1 is then supplied from Q1 via diode D1 rather than from groundvia R_(o) and diode D2.

Current source 22 is a "mirror image" of current source 20, and isadapted to draw current I4 through diode D5 when control current Ic ison and to draw no current through diode D5 when control current Ic isoff. Current source 22 includes a PNP transistor Q3 and an NPNtransistor Q4 which form another "quasi-thyristor" switch 26 controlledby control current Ic applied to the base of Q4. The negative voltagesource -E is applied to the emitter of transistor Q4. The base of Q4 isconnected to the -E source through a resistor R8 and to the collector oftransistor Q3. The base of transistor Q3 is connected to ground througha resistor R11, to a positive voltage source +E through a resistor R12,and to the collector of transistor Q4. A resistor R6 couples the emitterof transistor Q3 to a positive voltage source while an inductor L2couples the transistor Q3 emitter to capacitor C4.

When Ic is off, the base of transistor Q4 is low and transistor Q4 isoff. The base voltage of transistor Q3 is determined by the resistordivider R11, R12 so that Q3 is conducting a constant current determinedby R6. This current produces a voltage drop across R8 but not highenough to turn on the transistor Q4. In steady state, the current I4through inductor L2 is zero. The +E voltage source acting throughresistor R6 pulls V2 up to reverse bias diode D3 and to forward biasdiode D4 so that current I2 is supplied to resistor R_(o). When currentIc is turned on, the base of transistor Q4 is pulled up and transistorQ3 turns on. A negative-going voltage pulse appears at the emitter oftransistor Q3 as it turns on and this pulse is integrated by inductor L2to produce current ramp I4 which is drawn through capacitor C4 andthrough diode D5 in the reverse direction. When diode D5 switches toreverse bias state, voltage V2 is driven low to forward bias diode D3and reverse bias diode D4. Current I2 is then transmitted throughtransistor Q3 via diode D3 rather than through load resistor R_(o) viadiode D4.

Transmission lines T1 and T2 are of matched impedance and length and theresistance of resistors R1 and R2 match the impedance of thetransmission lines so that the transmission lines are terminated withtheir characteristic impedance. Thus control voltage pulses V1 and V2are transmitted over transmission lines T1 and T2 and are forwarded toground through capacitors Cl and C2 without reflections. Capacitors C3and C4 are provided to block the DC biasing current I3.

The rise time of the voltages appearing at the emitters of transistorsQ1 and Q3 is about 1 nanosecond, much slower than the rise time ofcontrol voltages V1 and V2 when the step-recovery diode D5 switches toreverse bias state. It is therefore important to ensure that V1 does notrise enough to forward bias diode D1 until diode D5 switches state.Therefore inductors L1 and L2 are sized to appropriately limit thevoltage developed across the step-recovery diode D5 after transistors Q1and Q3 switch on, while the diode D5 is still conducting in the reversedirection. This ensures that switches 16 and 18 do not change switchingstate until the step-recovery diode switches.

Thus the pulse generator of the present invention provides an outputvoltage pulse of adjustable magnitude and polarity having an abrupttransition to zero potential. The use of the pair of diode switches 16and 18 controlled by balanced control voltages V1 and V2 to switchopposing load currents away from a load resistor R_(o) permits the pulsegenerator 10 of the present invention to drive the output voltage Vo toa flat ground potential while eliminating any ringing in Vo followingthe transition due to ringing currents transmitted through inherentcapacitances associated with diode switches 16 and 18. The use of thestep-recovery diode D5 to control the switching of control voltages V1and V2 ensures that the transition to ground potential of the outputvoltage Vo is very rapid.

While a preferred embodiment of the present invention has been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

We claim:
 1. An apparatus for producing an output voltage stepcomprising:means for producing a first current; means for producing asecond current, the first and second currents being of oppositepolarities and unequal magnitudes; means for generating first and secondcontrol signals wherein the first and second control signals areswitched between first and second voltage states, the first and secondvoltage states being of equal magnitude but opposite polarity; a loadresistor; first switch means responsive to the first control signal fordirecting the first current through the load resistor when the firstcontrol signal is of the first voltage state and for preventing thefirst current from passing through the load resistor when the firstcontrol signal is of the second voltage state; and second switch meansresponsive to the second control signal for directing the second currentthrough the load resistor when the second control signal is of thesecond voltage state and for preventing the second current from passingthrough the load resistor when the second control signal is of the firstvoltage state, such that when the first control signal is of the firstvoltage state and the second control signal is of the second voltagestate, the first and second currents are directed through the loadresistor and an output voltage of magnitude proportional to thedifference in magnitudes between the first and second currents isdeveloped across the load resistor, and such that when the first controlsignal is of the second voltage state and the second control signal isof the first voltage state, the first and second currents are directedaway from the load resistor and no output voltage is developed acrossthe load resistor in response to the first and second currents.
 2. Anapparatus for producing an output voltage step comprising:means forproducing a first current; means for producing a second current, thefirst and second currents being of opposite polarities and unequalmagnitudes; means for generating first and second control signalswherein the first and second control signals are switched between firstand second voltages states, the first and second voltage states being ofequal magnitude but opposite polarity; a load resistor; first switchmeans responsive to the first control signal for directing the firstcurrent through the load resistor when the first control signal is ofthe first voltage state and for preventing the first current frompassing though the load resistor when the first control signal is of thesecond voltage state; and second switch means responsive to the secondcontrol signal for directing the second current through the loadresistor when the second control signal is of the second voltage stateand for preventing the second current from passing through the loadresistor when the second control signal is of the first voltage state,such that when the first control signal is of the first voltage stateand the second control signal is of the second voltage state, the firstand second currents are directed through the load resistor and an outputvoltage of magnitude proportional to the difference in magnitudesbetween the first and second currents is developed across the loadresistor, and such that when the first control signal is of the secondvoltage state and the second control signal is of the first voltagestate, the first and second currents are directed away from the loadresistor and no output voltage is developed across the load resistor inresponse to the first and second currents, wherein said first switchmeans comprises: a first diode having an anode and a cathode; and asecond diode having an anode and a cathode, the cathode of the seconddiode being connected to the cathode of the first diode, said firstcontrol signal being applied to the anode of the first diode, the anodeof the second diode being connected to the load resistor, said firstcurrent being applied to the cathodes of said first and second diodes.3. The apparatus according to claim 2 wherein said second switch meanscomprises:a third diode having an anode and a cathode; and a fourthdiode having an anode and a cathode, the anode of the fourth diode beingconnected to the anode of the third diode, said second control signalbeing applied to the cathode of the third diode, the cathode of thefourth diode being connected to the load resistor, said second currentbeing applied to the anodes of the third and fourth diodes.
 4. Theapparatus according to claim 3 wherein the first, second, third andfourth diodes comprise Schottky diodes.
 5. The apparatus according toclaim 3 wherein the means for generating first and second controlsignals comprise:a fifth diode having an anode and a cathode; and meanssupplying control current selectively in one of reverse and forward biasdirections through said fifth diode for switching the fifth diodebetween forward and reverse bias states, the cathode of the fifth diodebeing coupled to the anode of the first diode and the anode of the fifthbeing coupled to the cathode of the third diode.
 6. The apparatusaccording to claim 5 wherein the fifth diode comprises a step-recoverydiode.
 7. The apparatus according to claim 6 wherein the first, second,third and fourth diodes comprise Schottky diodes.
 8. An apparatus forproducing an output voltage step comprising:means for producing a firstcurrent; means for producing a second current, the first and secondcurrents being of opposite polarities and unequal magnitudes; a loadresistor; a first diode having an anode and a cathode; a second diodehaving an anode and a cathode, the cathode of the second diode beingconnected to the cathode of the first diode, the anode of the seconddiode being connected to the load resistor, said first current beingapplied to the cathodes of said first and second diodes; a third diodehaving an anode and a cathode; a fourth diode having an anode and acathode, the anode of the fourth diode being connected to the anode ofthe third diode, the cathode of the fourth diode being connected to theload resistor, said second current being applied to the anodes of thethird and fourth diodes; a fifth diode having an anode and a cathode,the cathode of the fifth diode being coupled to the anode of the firstdiode and the anode of the fifth being coupled to the cathode of thethird diode; and means supplying control current selectively in one ofreverse and forward bias directions through said fifth diode forswitching the fifth diode between forward and reverse bias states, suchthat when the fifth diode is forward biased, the first and third diodesare reverse biased and the second and fourth diodes are forward biased,the second and fourth diodes thereby directing the first and secondcurrents through the load resistor such that an output voltage ofmagnitude proportional to the difference in magnitudes between the firstand second currents is developed across the load resistor, and such thatwhen the fifth diode is reverse biased, the first and third diodes areforward biased and the second and fourth diodes are reverse biased, thefirst and third diodes thereby directing the first and second currentsaway from the load resistor such that no output voltage is developedacross the load resistor in response to the first and second currents.9. The apparatus according to claim 8 wherein said means supplyingcontrol current selectively in one of reverse and forward biasdirections through said fifth diode for switching the fifth diodebetween forward and reverse bias states comprises:means for transmittinga constant third current through the fifth diode in a forward direction;and means for selectively transmitting a fourth current through thefifth diode in a reverse direction, said control current comprising thesum of said third and fourth currents.
 10. The apparatus according toclaim 9 wherein the fifth diode comprises a step-recovery diode.
 11. Theapparatus according to claim 10 wherein the first, second, third andfourth diodes comprise Schottky diodes.